Method and system for controlling frequency

ABSTRACT

A method and system for receiving a signal in a received frequency and shifting the received frequency to become a desired frequency is provided. The system includes a controllable oscillator for generating a first internal frequency, a frequency estimating unit connected to the controllable oscillator, a first frequency shift unit, connected to the controllable oscillator and to the frequency estimating unit, for shifting the received frequency according to the first internal frequency, thereby obtaining an initially shifted frequency and a second frequency shift unit connected to the first frequency shift unit and the frequency estimating unit for shifting the initially shifted frequency. The frequency estimating unit determines a total frequency shift value from the desired frequency, the received frequency and the first internal frequency and it also determines a first frequency shift value and a second frequency shift value from the total frequency shift value. Furthermore, the frequency estimating unit provides a first frequency change command to the controllable oscillator, so as to correct the first internal frequency according to the first frequency shift value, a second frequency change command to the second frequency shift unit, so as to shift the initially shifted frequency according to the second frequency shift value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of allowed U.S. patent application Ser. No. 08/910,091 filed Aug. 12, 1997, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method and system for controlling frequency in general and to a method and system for providing an appropriate frequency, within predetermined error limits.

BACKGROUND OF THE INVENTION

In the art, there are known methods and systems for controlling and moderating frequency. A conventional transceiver includes therein a local oscillator which, for example in radio frequency (RF) systems, is usually a frequency generating crystal. When the transceiver receives a signal in a given frequency f_(S), it has to shift the frequency f_(S) of the signal to a desired frequency f_(d) by producing an internal frequency f_(LO), using the local oscillator. Conventional frequency crystals are known to include a frequency shift which needs to be corrected, according to the received signal frequency f_(S).

Reference is now made to FIGS. 1 and 2. FIG. 1 is a schematic illustration of a prior art system, generally referenced 1, for controlling the internal frequency of a transceiver. FIG. 2 is a graphic illustration of the frequency versus voltage curve, referenced of a typical voltage-controlled-oscillator (VCO).

System 1 includes a voltage-controlled-oscillator 2 (VCO), an input frequency shift unit 5 and an output frequency shift unit 7, both connected to VCO 2, and a frequency estimator 4, connected to VCO 2 and to input frequency shift unit 5, for determining the required frequency correction.

Conventionally, a demodulator 6 is connected after the input frequency shift unit 5 and a modulator 8 is connected before the output frequency shift unit 7.

A received signal provided to the input frequency unit 5, incorporates a frequency f_(S). The VCO 2 generates an internal frequency f_(IN) and provides it to the input frequency shift unit 5. The input frequency shift unit 5 produces a new signal having an intermediate frequency of f_(I), wherein f_(I)=f_(S)−f_(IN), and provides the new signal to the frequency estimating unit 4 as well as to the demodulator 6. It will be appreciated that conventional demodulators define a maximal allowed frequency difference Δf_(ALLOWED) between f_(I) and f_(d), so that |f_(I)−f_(d)|≦Δf_(ALLOWED). A conventional oscillator oscillates at a frequency which changes due to poor manufacturing quality, temperature changes and the like so that a situation where |f_(I)−f_(d)|>Δf_(ALLOWED) often occurs.

The frequency estimator 4 detects the signal provided by the frequency shift unit 5. If |f_(I)−f_(d)|>Δf_(ALLOWED) then f_(I) has to be adjusted to equal f_(d). The frequency estimator 4 determines the frequency correction Δf=f_(d)−f_(I), determines a respective voltage V(Δf) and provides the voltage V(Δf) to the VCO 2. Accordingly, the VCO 2 generates a corrected internal frequency f_(IN).

It will be noted that, as long as |f_(I)−f_(d)|>Δf_(ALLOWED), the demodulator cannot properly process the incoming signal.

The modulator 8 produces a signal for transmitting at a frequency of f_(T) and provides the signal to the output frequency shift unit. 7. The VCO 2 provides an internal transmitting frequency f_(OUT) to the output frequency shift unit 7. In turn, the output frequency shift unit 7 produces a transmittable signal bearing a frequency of f_(OUT)+f_(T) which is the transmitted signal b broadcasting frequency.

It is noted that f_(IN) and F_(OUT) are generated around a main frequency f_(LO), generated by the local oscillator of the VCO unit 2.

Referring now to FIG. 2, when provided with a voltage at a value of v₁, the VCO 2 is expected to provide the respective frequency f. A conventional VCO, having an internal inaccuracy, may provide, for a given voltage value of v₁, a frequency in the range between f_(MIN) and f_(max). When maximum(f_(MAX)−f,f−f_(MIN))>Δf_(ALLOWED), the frequency correction cannot be deemed final.

Conventionally, after several cycles of detection and correction, the intermediate frequency f_(I) should differ from the desired frequency f_(d) by no more than Δf_(ALLOWED). Until then, the demodulator 6 cannot perform properly.

It will be appreciated that the modulator 8 is also dependent on the accuracy of the frequency provided thereto by the VCO 2.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a novel method and a novel system for providing an appropriate frequency, within predetermined error limits.

According to the present invention there is thus provided a system for receiving a signal in a received frequency and shifting the received frequency to become a desired frequency, the system including a controllable oscillator for generating a first internal frequency, a frequency estimating unit connected to the controllable oscillator, a first frequency shift unit, connected to the controllable oscillator and to the frequency estimating unit, for shifting the received frequency according to the first internal frequency, thereby obtaining an initially shifted frequency and a second frequency shift unit connected to the first frequency shift unit and the frequency estimating unit for shifting the initially shifted frequency.

The frequency estimating unit determines a total frequency shift value from the desired frequency, the received frequency and the first internal frequency and it also determines a first frequency shift value and a second frequency shift value from the total frequency shift value.

Furthermore, the frequency estimating unit provides a first frequency change command to the controllable oscillator, so as to correct the first internal frequency according to the first frequency shift value, a second frequency change command to the second frequency shift unit, so as to shift the initially shifted frequency according to the second frequency shift value.

The frequency estimating unit may provides the first frequency change command to the controllable oscillator as an electrical voltage, with respect to the controllable oscillator characteristics.

The second frequency shift unit can be a hardware unit or a software unit. The second frequency shift unit may include means for performing a frequency shift by multiplying the initially shifted frequency by e^(j2πΔf), wherein Δf is the second frequency shift value.

The controllable oscillator may generates a second internal frequency, for example, for use in a combined transmitter receiver implementations a. In such a case, the system further includes a first output frequency shift unit, connected to the controllable oscillator for shifting a transmittable signal according to the second internal frequency.

For providing enhanced frequency correction according to the invention, the system further includes a second output frequency shift unit connected before the first output frequency shift unit and to the frequency estimating unit. The frequency estimating unit provides a command to the second output frequency shift unit to shift the transmittable signal according to the second frequency shift value thereby producing an initially shifted transmittable signal. Then, the first output frequency shift unit shifts the initially shifted transmittable signal according to the second internal frequency.

According to another aspect of the present invention there is provided a method for operating a system generating a controllable internal frequency f_(IN), a method for shifting a signal from an initial frequency f_(S) towards a desired frequency f_(D) to a final frequency f_(F), wherein |f_(D)−f_(F)| is less than a predetermined allowed frequency error f_(ALLOWED).

The method includes the steps of:

determining a total frequency shift value F_(TOTAL) according to the internal frequency f_(IN) and the initial frequency value f_(S);

determining a first frequency shift value Δf₁ from the total frequency shift value f_(TOTAL) and the predetermined allowed frequency error f_(ALLOWED);

determining a second frequency shift value Δf₂ from the total frequency shift value f_(TOTAL) and the first frequency shift value Δf₁;

adjusting the internal frequency f_(IN) according to the first frequency shift value Δf₁ thereby shifting the initial frequency f_(S) so as to produce a shifted initial frequency f_(SHIFTED); and

shifting the shifted initial frequency f_(SHIFTED) according to the second frequency shift value Δf₂ thereby producing the final frequency f_(F).

The sum of the first frequency shift value Δf₁ and the second frequency shift value Δf₂ equals the total frequency shift value f_(TOTAL). The first frequency shift value Δf₁ defines a frequency error f_(ERROR) which is less than the predetermined allowed frequency error f_(ALLOWED). Since Δf₂ includes an insignificant frequency error, thus, according to the invention, it is guarantied that the total frequency correction equals frequency error f_(ERROR).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

FIG. 1 is a schematic illustration of a prior art system, for controlling the internal frequency of a transceiver;

FIG. 2 is a graphic illustration of the frequency versus voltage curve, referenced of a typical voltage-controlled-oscillator (VCO);

FIG. 3 is a schematic illustrating of a system, constructed and operative according to a preferred embodiment of the present invention; and

FIG. 4 is a schematic illustrating of a method for operating the system of FIG. 3, operative according to another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method of the present invention overcomes the disadvantages of the prior art by providing a combined hardware and software frequency correction.

Reference is now made to FIG. 3 which is a schematic illustrating of a system, generally referenced 100, constructed and operative according to a preferred embodiment of the present invention.

System 100 includes a voltage-controlled-oscillator 102 (VCO), a first input frequency shift unit 105 and a first output frequency shift unit 107, both connected to VCO 102, a frequency estimator 104, connected to VCO 102 and to first input frequency shift unit 105, for determining the required frequency correction, a second input frequency shift unit 109, connected to the frequency estimator 104 and to the first input frequency shift unit 105 and a second output frequency shift unit 111, connected to the frequency estimator 104 and to the first output frequency shift unit 107.

In the present example, a demodulator 106 is connected after the second input frequency shift unit 109 and a modulator 108 is connected before the second output frequency shift unit 111.

For each given voltage value V₁, the VCO 102 produces a frequency f_(LO)±f_(ERROR) due to the uncertainty incorporated in the ability of the VCO 102 to produce a predetermined frequency f_(LO) when provided with a predetermined voltage V₁. The allowed frequency change Δf_(ALLOWED) determines a maximum VCO frequency correction Δf_(MAX) where f_(ERROR)(Δf_(MAX))=Δf_(ALLOWED). For example, if Δf_(ALLOWED)=100 Hz and the maximal frequency error of the VCO 102 is ±25% then, a maximum frequency f_(MAX) which equals 400 Hz guarantees that |f_(ERROR)|≦100 Hz.

The present invention attempts to always provide the VCO 102 with a frequency change signal which is not greater than Δf_(MAX) such that the maximal frequency error never exceeds Δf_(ALLOWED).

It is noted that, in the present example, f_(IN) is equal to f_(LO) and f_(OUT) is derived therefrom f_(LO) using a frequency gap δ, where f_(out)=f_(LO)−δ.

A received signal, provided to the first input frequency unit 105, incorporates a frequency f_(S). The VCO 102 generates a frequency f_(IN) and provides the frequency f_(IN) to the first input frequency shift unit 105. The first input frequency shift unit 105 produces a new signal having a frequency f_(I) wherein f_(I)=f_(S)−f_(IN) and provides the new frequency f_(L) to the frequency estimating unit 104 as well as to the second input frequency shift unit 109.

Frequency estimating unit 104 is able to control the frequency both in hardware and in software. A hardware frequency change Δf_(HW) is generated by providing voltage to the VCO 102. A software frequency change Δf_(SW-IN) is generated by providing the second input frequency shift unit 109 with a command to multiply the signal produced the first input frequency shift unit 105 by e^(jΔω) ^(_(SW)) , wherein Δω_(SW)=2πΔf_(SW-IN). It will be noted that the software frequency correction is considered an exact correction, bearing insignificant error.

The frequency estimator 104 detects the signal provided by the first frequency shift unit 105. If |f_(d)−f_(I)|>Δf_(ALLOWED) then, f_(IN) has to be adjusted to equal f_(s)f_(d). The frequency estimator 104 determines the frequency correction Δf=f_(d)−f_(I). If |Δf|>f_(MAX) then frequency estimator 104 sets Δf_(HW) to be sign(Δf)Δf_(MAX) and Δf_(SW-IN) to be Δf−f_(MAX). Otherwise, the frequency estimator 104 sets Δf_(HW) to be Δf and Δf_(SW) _(—) _(IN) to be zero. The frequency estimator 104 further determines a voltage value correction ΔV_(VCO)=ΔV(Δf_(HW)) and provides it to the VCO 102. The VCO 102 generates a corrected internal frequency f_(IN).

At the same time, the frequency estimator 104 provides the value of Δf_(SW-IN) to the second input frequency shift unit 109. The second input frequency shift unit 109 shifts the signal received from the first input frequency shift unit 105 by Δf_(SW-IN). If, for example the second input frequency shift unit 109 is implemented in base-band then the frequency shift is performed by multiplying the signal received from the first input frequency shift unit 105 by e^(j2πΔf) ^(_(SW-IN)) .

Furthermore, the frequency estimator 104 provides a value of Δf_(SW-OUT) to the second output frequency shift unit 111. The second output frequency shift unit 111 shifts the signal (having a frequency f_(T)) received from the modulator 108 by Δf_(SW-OUT) and provides the resultant signal to the first output frequency shift unit 107. The first output frequency shift unit 107 receives frequency f_(OUT) from the VCO 102, shifts the frequency received from the second output frequency shift unit 111 by f_(OUT), so as to produce a transmitted signal bearing a frequency of f_(OUT)+f_(T)+Δf_(SW-OUT).

In a conventional VCO, f_(IN) and f_(OUT) can be determined according to a basic frequency f_(LO), wherein often f_(IN)−f_(LO)≠f_(LO)−f_(OUT). Hence Δf_(SW-OUT) does not necessarily equal Δf_(SW-IN) but can be determined accordingly, since f_(OUT) can be determined as f_(OUT)(f_(IN)).

It will be noted that a “software” frequency correction according to the present invention can be implemented using hardware means, for example, by providing a hardware multiplication unit followed by a low pass filter.

It will be noted that, according to the present invention, the total frequency correction provided to the demodulator 106 is equal to Δf_(SW-IN)+Δf_(HW), with an error f_(ERROR) which is no greater than Δf_(ALLOWED). Thus, the second input frequency shift unit 109 provides a signal to the demodulator 106 which withstands the predetermined frequency error limitations.

The same applies to the transmitted signal, provided by the first output frequency unit, which also includes an error f_(ERROR) which is not greater than Δf_(ALLOWED).

The result is that the system 100 is operative to transmit and receive according to fine tuned frequencies, within predetermined frequency error limitations, as soon as frequency estimator 102 provides Δf_(SW-IN) and Δf_(SW-OUT) for the first time.

It will be appreciated that, in conventional systems this is not possible until f_(IN) has stabilized within the error limitations which typically requires a few correction cycles and takes a significant amount of time.

After the corrected internal frequency f_(IN) has stabilized, the system 100 repeats the above operations until f_(IN) is corrected and stabilized whereas Δf<f_(ALLOWED).

Reference is now made to FIG. 4 which is a schematic illustration of a method for operating system 100, operative according to another preferred embodiment of the present invention.

In step 200, the system detects the internal hardware frequency f_(I), provided by the local oscillator, and the received signal frequency. Then, the system proceeds to step 202.

In step 202, the system 100 determines the following values and proceeds to step 204:

the required frequency correction value Δf wherein Δf=f_(D)−f_(I);

the hardware frequency correction value Δf_(HW) wherein Δf_(HW)=minimum(|Δf|, Δf_(MAX))sign(Δf). Δf _(MAX) is the maximal value of the VCO frequency which provides a frequency error no greater than the allowed frequency shift Δf_(ALLOWED). Δf_(ALLOWED) may be determined by the utilized communication standard, the construction of the transceiver and the like; and

the software frequency correction value Δf_(SW), wherein Δf_(SW=Δf−Δf) _(HW).

In step 204, the system 100 determines a voltage value V_(vco) wherein V_(vco)=V_(vco +V(Δf) _(HW)), provides the new value of V_(vco) to the VCO 102 and proceeds to step 206.

In step 206, the system 100 waits for f_(LO) to stabilize. It will be appreciated that until f_(LO) stabilized, it is virtually imposible to detect any effect of V_(vco) on f_(LO). After f_(LO) stabilized, the system 100 proceeds to step 208.

In step 208, if |f_(I)−f_(D) |<Δf_(THRESHOLD), then the system 100 proceeds to step 210. Otherwise, the system proceeds back to step 200 to repeat the operation.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow. 

What is claimed is:
 1. A method for correcting frequency comprising: modifying a received frequency by applying a first signal to generate a second signal having an adjusted frequency; generating a pair of frequency change commands based on the adjusted frequency of the second signal; providing a first of the pair of frequency change commands to modify a frequency of the first signal; and providing a second of the pair of frequency change commands to shift the adjusted frequency of the second signal.
 2. The method of claim 1, further comprising determining a total frequency shift value.
 3. The method of claim 2, further comprising determining the pair of frequency change commands from the total frequency shift value.
 4. The method of claim 1, further comprising shifting the adjusted frequency of the second signal in a hardware unit.
 5. The method of claim 1, further comprising shifting the adjusted frequency of the second signal in a software unit.
 6. A system for shifting a received frequency comprising: a first frequency shift unit to initially shift the received frequency to a shifted frequency; a frequency estimator which determines the shifted frequency and generates a plurality of frequency change commands based on the shifted frequency, wherein a first of the plurality of frequency change commands modifies the first frequency shift unit; and a second frequency shift unit which receives a second of the plurality of frequency change commands to shift the initially shifted frequency.
 7. The system of claim 6, wherein the second frequency shift unit determines the required shift of the initially shifted frequency from a total frequency shift value.
 8. The system of claim 6, wherein the second frequency shift unit is a hardware unit.
 9. The system of claim 6, wherein the second frequency shift unit is a software unit. 